Nothing is wrong with SDN and improvements do not need to necesarrily mean that anything is wrong in the first place.
That said, Proxmox VE's SDN is still an opt-in technology preview, so not covered by the enterprise support and generally meaning that it may have some subtle issues/rough edges around - albeit, the main author is rather quick to fix most stuff
So improving SDN support means, further improve documentation, code base and do extended tests so that we can mark it as stable.